This is an improvement over the machine of my prior patent, supra, wherein tennis balls are sequentially fed through a flexible, normally leaky flap valve into a plenum chamber and discharged therefrom through a barrel. Choking of the ball in the barrel raises the pressure in the plenum chamber and forces the flap sealingly against the chamber wall and thereby creates a sudden rise in air pressure behind the ball.
Throughout this art, various means have been devised for imitating the wiles of a top-flight server, for example, friction strips have been placed in the barrel for applying spin to the ball and crank and link mechanisms (see Balka, Jr., supra) have been used to oscillate a barrel about vertical and horizontal axes. By and large, these devices all suffer, in greater or lesser degree, from regularity such that the player to whom the ball is served can anticipate the variations in height or direction which the ball partakes. The object now is to provide a ball throwing machine having virtually infinite variations in throwing direction and ball spin, so that the player who is to return the ball never knows whether the ball will come at him high or low, to the right or left or with forespin, backspin, or bounce to his left or right.
Towards this end it is intended now to provide a tennis ball throwing machine wherein a barrel through which a ball is propelled not only has a spin-imparting Velcro strip on one side of its inner wall, but also has a mechanism for rotating the barrel about its lengthwise axis so that the type of spin (fore, back, etc.) varies cyclically and wherein the end of the barrel is bent so as to partake of an orbital motion as it is rotated.